In CATV (hereinafter sometimes cable TV or cable), the return band (hereinafter sometimes upstream) describes the frequency range allocated for transmission of signals from the subscriber premises back to the cable company. In typical designs, this frequency range is from 5 MHz to 42 MHz, 5 MHz to 65 MHz, 5 MHz to 85 MHz, or in the not-too-distant future 5 MHz to 200 MHz (see FIG. 1). Typical signals coming from the subscriber premise include, for example, set top box DVR/On Demand requests, test equipment data channels, and Data Over Cable Service Interface Specification (“DOCSIS”) Internet Protocol output cable modem carriers.
As part of performing an installation or servicing a CATV system, it is sometimes necessary to check the linearity in the downstream in CATV systems. In the past, to determine linearity in the downstream, installers and technicians have compared the amplitudes of downstream channels as they installed, or were troubleshooting, a subscriber's installation, and either adjusted the levels accordingly or otherwise determined what was responsible for the amplitude differences and took whatever steps were necessary to correct them.
In the past, to maintain linearity in the upstream in CATV systems, test equipment has transmitted swept carriers upstream to a signal analyzer constructed for this purpose, such as, for example, the Trilithic SST-9581, and monitored these carriers. Again, differences in amplitude were corrected by whatever means were appropriate under the circumstances. The sweep of the carriers had to avoid active upstream channels in the 5 MHz to 42 MHz bandwidth, or 5 MHz to 65 MHz bandwidth, 5 MHz to 85 MHz bandwidth, whichever is being used for upstream communication.
A significant problem with this scheme for the future is that the active upstream bandwidth is filling up with content. As a result, the sweeping carrier cannot appear in many places in the upstream band without risking interfering with upstream-bound signals. There are, however, the active upstream channels themselves.